Apparatus for controlling the voltage applied to an electrostatic shield used in a plasma generator

ABSTRACT

An apparatus for controlling the voltage applied to a shield interposed between an induction coil powered by a power supply via a matching network, and the plasma it generates, comprises a shield, a first feedback circuit, and a second feedback circuit. The power supply powers the shield. The first feedback circuit is connected to the induction coil for controlling the power supply. The second feedback circuit is connected to the shield for controlling the voltage of the shield. Both first and second feedback circuits operate at different frequency ranges. The first feedback circuit further comprises a first controller and a first sensor. The first sensor sends a first signal representing the power supplied to the inductive coil to the first controller. The first controller adjusts the power supply such that the power supplied to the inductor coil is controlled by a first set point. The second feedback circuit further comprises a second sensor, a second controller, and a variable impedance network. The shield is powered via a variable impedance network. The second sensor sends a second signal representative of the voltage of the shield to the second controller. The second controller adjusts the variable impedance network such that the voltage of the shield is controlled by a second set point.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to co-pending U.S. patent applicationSer. No. 09/676,462, filed on Sep. 29, 2000, which is acontinuation-in-part of co-pending U.S. patent application Ser. No.09/608,883, filed on Jun. 30, 2000, which is, in turn, acontinuation-in-part of co-pending Japanese Patent Application SerialNo. 2000-99728, filed on Mar. 31, 2000, in Japan. These patentapplications are not admitted to be prior art by the Applicant.

FIELD OF THE INVENTION

The present invention relates to a plasma processing apparatus. Moreparticularly, the present invention relates to a method and apparatusfor controlling the voltage applied to an electrostatic shield disposedbetween an induction coil and a plasma.

BACKGROUND OF THE INVENTION

Inductively coupled type dry etching systems are commonly used in thesemiconductor manufacturing industry. The dry etching apparatusgenerally has a process chamber with a ceiling of a dielectric wall, onwhich an annular or spiral radio frequency (RF) antenna is disposed.

There are several known techniques for exciting a plasma with RF fieldsincluding capacitive coupling, inductive coupling and wave heating. In astandard inductively coupled plasma (ICP) generator, RF current passingthrough a coil induces electromagnetic currents in a plasma. Thecurrents heat the conducting plasma by ohmic heating, so that it issustained in steady state. Typically, the current through a coil acts asthe primary winding of a transformer. The plasma acts as a single turnsecondary winding of the transformer.

During a plasma process, the RF antenna and a plasma excited in theprocess chamber are coupled not only inductively but also capacitively.Consequently, the inner surface of the dielectric wall made of, e.g.,quartz, which is exposed to the interior atmosphere of the processchamber near the RF antenna, is charged with a negative bias relative tothe plasma. With the potential difference between the plasma and theexposed inner surface of the dielectric wall, positive ions in theplasma collide with the exposed inner surface while being accelerated.As a result, problems arise in that contaminants are produced in theprocess chamber and the dielectric wall is worn off quickly.

In order to cope with these problems, a conductive Faraday shield isnormally disposed between a dielectric window and an insulating layerunder an RF antenna. The capacitive coupling between the RF antenna andthe plasma is disrupted by the Faraday shield, so that the exposed innersurface of the dielectric wall is protected from collisions withaccelerated positive ions from the plasma. The Faraday shield ispreferably connected to a source of RF potential to control its relativepotential or bias. One option is to connect the shield to a point fromthe antenna in which case the shield operates at the same frequency asthe coil as illustrated in FIG. 1. However, the current that is coupledout of the coil reduces the magnetic coupling with the plasma. Althoughsuch a powering scheme is simple and efficient, the controls of recipevariables are therefore limited.

An alternative powering scheme uses an external second auxiliary RFpower supply separate from the main supply that powers the coil asillustrated in FIG. 2. The advantages of this scheme are the option tooperate the shield at a different frequency from the antenna, thesubstantially smaller interaction between the shield and the coilcircuit, and simplicity of control. However such extra circuitry resultsin higher cost and may complicate control.

Accordingly, a need exists for an apparatus and method to independentlycontrol the antenna current and the Faraday shield voltage both poweredby a single power supply.

BRIEF DESCRIPTION OF THE INVENTION

An apparatus for controlling the voltage applied to a shield interposedbetween an induction coil powered by a power supply via a matchingnetwork, and the plasma it generates, comprises a shield, a firstfeedback circuit, and a second feedback circuit. The power supply powersthe shield. The first feedback circuit is connected to the inductioncoil for controlling the power supply. The second feedback circuit isconnected to the shield for controlling the voltage of the shield. Bothfirst and second feedback circuits operate at different frequencyranges. The first feedback circuit further comprises a first controllerand a first sensor. The first sensor sends a first signal representingthe power supplied to the inductive coil to the first controller. Thefirst controller adjusts the power supply such that the power suppliedto the inductor coil is controlled by a first set point. The secondfeedback circuit further comprises a second sensor, a second controller,and a variable impedance network. The shield is powered via a variableimpedance network. The second sensor sends a second signalrepresentative of the voltage of the shield to the second controller.The second controller adjusts the variable impedance network such thatthe voltage of the shield is controlled by a second set point.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

In the drawings:

FIG. 1 is a schematic diagram illustrating an antenna and a Faradayshield powered by a single power supply according to a prior art;

FIG. 2 is a schematic diagram of an antenna powered by a first powersupply and a Faraday shield powered by a second power supply accordingto a prior art;

FIG. 3 is a schematic diagram illustrating an antenna and a Faradayshield both powered by a single power supply according to a specificaspect of the present invention;

FIG. 4 is a flowchart illustrating a method to independently control thepower supplied to an antenna and the voltage supplied to a shield from asingle power supply according to a specific aspect of the presentinvention; and

FIG. 5 is a flowchart illustrating an algorithm of a power controlleraccording to a specific aspect of the present invention.

FIG. 6 is a flowchart illustrating an algorithm of a voltage controlleraccording to a specific aspect of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are described herein in the contextof an apparatus and method for controlling the voltage applied to anelectrostatic shield disposed between an induction coil and the plasmait is used to generate. Those of ordinary skill in the art will realizethat the following detailed description of the present invention isillustrative only and is not intended to be in any way limiting. Otherembodiments of the present invention will readily suggest themselves tosuch skilled persons having the benefit of this disclosure. Referencewill now be made in detail to implementations of the present inventionas illustrated in the accompanying drawings. The same referenceindicators will be used throughout the drawings and the followingdetailed description to refer to the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

In accordance with the present invention, the components, process steps,and/or data structures may be implemented using various types ofoperating systems, computing platforms, computer programs, and/orgeneral purpose machines. In addition, those of ordinary skill in theart will recognize that devices of a less general purpose nature, suchas hardwired devices, field programmable gate arrays (FPGAs),application specific integrated circuits (ASICs), or the like, may alsobe used without departing from the scope and spirit of the inventiveconcepts disclosed herein.

FIG. 3 is a schematic diagram illustrating an antenna 302 and a Faradayshield 304 both powered by a single power supply 306 according to aspecific aspect of the present invention. The power supply 306preferably generates a radio frequency (RF) current to the antenna 302,for example in the shape of a spiral coil, via a matching network 308. Afirst feedback circuit comprises a sensor 310 electrically connected toa power controller 312. The sensor 310 may be any sensors capable ofmeasuring power or current. The sensor is coupled, preferablymagnetically coupled, to one of the power supply lines 314 thatelectrically connects the matching network 308 with the antenna 302. Thepower controller 314 monitors the current or power flowing into theantenna 302 and based on the signal received from the sensor 310 adjuststhe power supply 306 to generate a desired current or power. This signalcan be used as a feedback to compare with an external setpoint 316,and/or the matching network 308 settings, such that the current or powerflowing in the antenna is held stable. Alternatively, the matchingnetwork 308 can be allowed to track the antenna variations, using astandard auto-tuning algorithm, or it can be held fixed at a fixedvalue, or switched to preset fixed values. Further details about thealgorithms are provided below in FIG. 5.

It should be noted that the Faraday shield represents any metal platethat acts as a Faraday shield. The shield 304 is disposed between theantenna 302 and a TCP window (not shown) above a plasma chamber (notshown) and is substantially parallel to the TCP window. The shield 304is powered by tapping energy from the antenna 302. Essentially theshield 304 is run in parallel with the antenna 302 circuit seen as aload, picking up a high voltage point at either the input or terminationend of the antenna 302. The shield 304 is preferably connected via avariable impedance component or network 318, preferably a motor drivenvacuum capacitor. A sensor 320, preferably a voltage sensor, isconnected at the shield feed point after the series control element 318,such that the shield voltage can be measured and used a feedback controlvariable. Optionally, the current or power may be measured at the samesensing point. The sensor 320 measures the voltage of the shield 304 andsends a signal representative of the measured voltage to a voltagecontroller 322 forming another feedback circuit. The voltage controller322 compares the voltage signal with an external setpoint 324. Theoutcome of the comparison generates a signal to the variable impedancecomponent 318 to adjust the voltage of the shield 304. The algorithm ofthe voltage controller 322 is further described below in FIG. 6.

If the network is not accurately matched dynamically to the total load,i.e. the antenna 302 circuit and the shield 304 circuit, there will be ameasure of reflected power. However, as long as the efficiency does notreduce excessively, and as long as the power supply 306 can tolerate thereflected power, the reflected power effect can be neglected.

The feedback algorithms could be achieved using analog circuitry, ormore likely by imposing a digital controller into the analogelectronics. As described previously, the apparatus operates twofeedback loops while controlling two dependent variables simultaneously,without any priority being set. In order to ensure that the two feedbackloops can operate essentially independently, it is necessary to set up adominant pole in the response by separating the two feedback loops inthe frequency domain. That is one feedback circuit may have a fasterresponse rate while the other may have a slower response rate. It ispreferable to stabilize the current in the antenna 302 on a fast timescale, for example, from 1 kHz to 1 MHz, to improve plasma stability. Itis preferable to stabilize the voltage in the shield 304 on a relativelyslower time scale, for example 10 Hz to 100 Hz. Otherwise, theinteraction may be so strong as to keep the system in permanent chaoticoscillation.

FIG. 4 is a flow diagram 400 illustrating a method to independentlycontrol the power supplied to the antenna 302 and the voltage suppliedto the shield 304 from the single power supply 306 according to aspecific aspect of the present invention. In a first block 402, thepower supply 306 supplies an RF current to the antenna 302 via amatching network 308 as illustrated in FIG. 3. In block 404, the shield304 is powered by tapping energy from the antenna 302 via a variableimpedance network 318. In a block 406, the sensor 310 measures thecurrent supplied to the antenna 302 and sends a signal to the powercontroller 312 representative of the power or current passing throughone of the power lines 314. It should be noted that the antenna currentcan be monitored at the antenna input, output, a combination of both, orat some integrated distributed value as required by the losses andstanding waves along the antenna. In a block 408, the power controller312 compares the signal from the sensor 310 with an external setpoint316 and adjusts the power supply to stabilize the current in the antenna302. Both blocks 406 and 408 operate at a relatively fast response rate,i.e. at a higher frequency.

In a block 410, the voltage of the shield 304 is measured using a sensor320, such as a voltage sensor, after the series control element, i.e.the variable impedance component 318. The sensor 320 sends a signalrepresenting the voltage of the shield 304 to the voltage controller322. In block 412, the voltage controller 412 compares the signal fromthe sensor 320 with an external setpoint 324 and adjusts the variableimpedance component 318 to stabilize the voltage in the shield 304. Bothblocks 410 and 412 operate at a relatively slow response rate, i.e. at alower frequency.

Both blocks 406 and 408 along with blocks 410 and 412 operatessimulateously but at different response rates. It should be noted thatanother feedback control variable that can be measured is the outputpower of the power supply and measured load impedance. Thesemeasurements at the output of the power supply are generally used toauto-tune or set the matching network 308 and level the delivered powerto the antenna 302.

The preferred embodiment would regard the antenna 302 current or powerand the shield 304 voltage as independent setpoints, and measured valuesof the corresponding output parameters as the primary feedbackvariables. Control would be implemented by adjusting the power supplysetpoint 326 to stabilize the current or power of the antenna 302 andadjusting the variable impedance network 318 to stabilize the voltage ofthe shield 304. The matching network values can then be automaticallyadjusted to optimize power transfer, or at least to maintain the loadseen by the power supply 306 within acceptable operating range asdetected by the power supply output sensors (not shown). The desiredstanding wave pattern in the antenna 302 can be set by adjusting itstermination impedance to the ground. It can therefore be appreciatedthat the present invention allows independent stable control of thecurrent in the antenna and the voltage of the shield, even if the loadvaries. This is accomplished while sharing a single power supply and amatching network.

FIG. 5 is a flowchart illustrating an algorithm of a power controlleraccording to a specific aspect of the present invention. In block 502, auser sets a desired power as an independent setpoint in the powercontroller. In block 504, the current in the antenna is measured withany suitable sensor. In block 506, the current measured is compared withthe independent setpoint and the power supply is adjusted accordinglysuch that the power is stabilized as the desired power.

FIG. 6 is a flowchart illustrating an algorithm of a voltage controlleraccording to a specific aspect of the present invention. In a firstblock 602, a user sets a desired voltage as an independent setpoint. Inblock 604, the shield voltage is measured with any suitable sensor. Inblock 606, the measured shield voltage is compared with the independentsetpoint and the shield voltage is adjusted accordingly by adjusting theseries impedance network to stabilize the voltage.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art having thebenefit of this disclosure that many more modifications than mentionedabove are possible without departing from the inventive concepts herein.The invention, therefore, is not to be restricted except in the spiritof the appended claims.

What is claimed is:
 1. An apparatus for controlling the voltage appliedto a shield interposed between an induction coil powered by a powersupply via a matching network, and the plasma it generates, saidapparatus comprising: a shield powered by the power supply, said shieldcoupled to the induction coil with a variable impedance network; a firstfeedback circuit connected with the induction coil for controlling thepower supply; and a second feedback circuit connected with said shieldfor controlling the voltage of said shield with said variable impedancenetwork.
 2. The apparatus according to claim 1 wherein said firstfeedback circuit and said second feedback circuit operate at differentfrequency ranges.
 3. The apparatus according to claim 2 wherein saidfirst feedback circuit operates at a frequency in a range of about 1 kHzto about 1 MHz.
 4. The apparatus according to claim 2 wherein saidsecond feedback circuit operates at a frequency in a range of about 10Hz to about 100 Hz.
 5. The apparatus according to claim 1 wherein saidfirst feedback circuit further comprises a first controller and a firstsensor.
 6. The apparatus according to claim 5 wherein said first sensorsends a first signal representing the power supplied to the inductivecoil to said first controller.
 7. The apparatus according to claim 6wherein said first controller adjusts the power supply such that thepower supplied to the inductor coil is controlled by a first set point.8. The apparatus according to claim 1 wherein said second feedbackcircuit further comprises a second sensor, a second controller, and saidvariable impedance network.
 9. The apparatus according to claim 8wherein said shield is powered via said variable impedance network. 10.The apparatus according to claim 9 wherein said variable impedancenetwork further comprises a variable capacitor.
 11. The apparatusaccording to claim 10 wherein said second sensor sends a second signalrepresentative of the voltage of said shield to said second controller.12. The apparatus according to claim 11 wherein said second controlleradjusts said variable impedance network such that the voltage of saidshield is controlled by a second set point.
 13. An apparatus forcontrolling the voltage applied to a shield interposed between aninduction coil powered by a power supply through a matching network anda plasma it generates, the apparatus comprising: a variable impedancenetwork coupled to the induction coil; a shield coupled to said variableimpedance network, said shield powered by the power supply; a firstsensor connected with said shield for measuring the voltage of saidshield, said first sensor sending a first signal representative of themeasured voltage; and a first controller connected with said sensor forreceiving said first signal, said controller adjusting said variableimpedance network such that said first signal is close to a firstspecified set point; a second sensor connected with the induction coilfor measuring power flowing in the induction coil, said second sensorsending a second signal representative of the measured power; and asecond controller connected with said second sensor for receiving saidsecond signal, said second controller adjusting the power supply suchthat said second signal is close to a second specified set point. 14.The apparatus according to claim 13 wherein said variable impedancenetwork further comprises a variable vacuum capacitor.
 15. The apparatusaccording to claim 13 wherein said first controller operates at afrequency in a range of about 1 kHz to about 1 MHz.
 16. The apparatusaccording to claim 13 wherein said second controller operates at afrequency in a range of about 10 Hz to about 100 Hz.
 17. An apparatusfor controlling the voltage applied to a shield interposed between aninduction coil powered by a power supply via a matching network, and theplasma it generates, said apparatus comprising: means for powering theshield, wherein a variable impedance network couples the shield to theinduction coil; means for adjusting the power supply based on a firstfeedback circuit; and means for adjusting the voltage of the shieldbased on a second feedback circuit.
 18. The apparatus according to claim17 wherein said first feedback circuit and said second feedback circuitoperate at different frequency ranges.
 19. The apparatus according toclaim 18 wherein said first feedback circuit operates at a frequency ina range of about 1 kHz to about 1 MHz.
 20. The apparatus according toclaim 18 wherein said second feedback circuit operates at a frequency ina range of about 10 Hz to about 100 Hz.
 21. The apparatus according toclaim 17 wherein said first feedback circuit further comprises a firstcontroller and a first sensor.
 22. The apparatus according to claim 21wherein said first sensor sends a first signal representing the powersupplied to the inductive coil to said first controller.
 23. Theapparatus according to claim 22 wherein said first controller adjuststhe power supply such that the power supplied to the inductor coil iscontrolled by a first set point.
 24. The apparatus according to claim 17wherein said second feedback circuit further comprises a second sensor,a second controller, and said variable impedance network.
 25. Theapparatus according to claim 24 wherein said shield is powered via saidvariable impedance network.
 26. The apparatus according to claim 25wherein said variable impedance network further comprises a variablecapacitor.
 27. The apparatus according to claim 24 wherein said secondsensor sends a second signal representative of the voltage of saidshield to said second controller.
 28. The apparatus according to claim24 wherein said second controller adjusts said variable impedancenetwork such that the voltage of said shield is controlled by a secondset point.